This invention relates to a device for the production of sol-gel globules, especially green sol-gel globules for sphere-pac nuclear fuel, by which at least one each of coarse and fine globule fractions are produced by the dripping of sol-liquid into a vertical gelling stream for producing the coarse fraction, and by the scattering of at least one jet of the sol-gel liquid in a horizontal gelling stream for producing the fine fractions, in an optional sequence of operations.
Sphere-pac nuclear fuel is defined as a mixture of discrete globule sizes. It is evidence of the quality of fuel elements in general when the fuel is evenly distributed with even density throughout the wrapper tube. This requirement can be satisfied for sphere-pac nuclear fuel if the globules are of possible uniform size within a particular fraction. Generally, two to three globular fractions are needed for sphere-pac nuclear fuel, their diameters having a relationship of 1:7 to 1:10.
For the production of sol-gel globules, small globular droplets of an aqueous metal solution having gelling properties, the so-called mixing feed solution, are gelled within a stream of a hot gelling liquid which does not mix with the mixing feed solution as, for example, silicon fluid, because of an internal chemical reaction, so that they no longer adhere to each other and are carried away by the gelling stream. With this process, aluminum--or iron globules, for example, but in particular also the oxides thereof--and carbide nuclear fuels may be produced.
For nuclear fuels, it is customary to utilize uranium with varying plutonium and/or thorium contents, or thorium alone as the heavy metal. The aqueous solution of nitrates of these metals contains an ammonia donor, for example, hexamethylentetramine, which quickly decays when heated, very rapidly raising the pH value of the solution which leads to the amorphous or microcrystalline precipitation of the heavy-metal hydroxides.
Sol-gel globules with a diameter of more than 1 mm can be produced with satisfactory throughput and a very narrow dimensioning by dripping sol-liquid through one or more hollow needles into a hot gelling stream slowly flowing within a vertical gelling column. The spherical feed droplets gel during their descent within the gelling stream and, given a suitable relationship between the densities of the gelling means and the sol-liquid, the droplets are also sufficiently solidified in their spherical shape. The gelling column may comprise, for example, a double-walled tube open at the top and preferably made of glass, the outer tube having an enlarged upper end to form an overflow container with a discharge spout, and the lower end of the outer tube being connected to the inner tube becomes a discharge. The gelling column is in precisely vertical adjustment, so that the sol-liquid droplets, issuing from the hollow needle or needles entering the gelling stream descending within the vertical gelling column, may descend within such stream without touching the wall of the inner tube, and whereby the solidified sol-gel globules may be transferred outwardly of the gelling column through the discharge opening, together with the gelling liquid. For continuous operation, the discharge is connected with a separating device, containing strainers, where the sol-gel globules are separated from the gelling liquid and the latter is directly returned into the heatable vat of gelling liquid. From this vat, the collected and reheated gelling liquid is cleaned by filters, regenerated and then pumped back into the gelling column. The hollow needles are disposed a sufficient distance above the gelling column, so that a complete separation of the undivided sol-liquid from the hot gelling stream is assured thereby resulting in a trouble-free operation.
Smaller sol-gel globules with diameters less than one-half a millimeter cannot be satisfactorily produced by this dripping method, since even with the use of several hollow needles, the throughput of one gelling column alone would be insufficient. German Offenlegungschrifft No. 24 24 209 (Interfuel 1974) describes a production method for small sol-gel globules with satisfactory throughputs and with sufficiently small deviations in size. The device used with such a system is known as "jet-chopper". One or more thin jets of sol-liquid in their free, laminar state are introduced more or less vertically, into a horizontal, ribbon shaped and free flowing laminar, hot gelling stream, where each of the feed jets disintegrates into regular, globule droplets which are solidified almost immediately in this hot gelling liquid thereby forming corresponding regular globules. The two flows are maintained by the gelling liquid being delivered through a ribbon nozzle, and sol-liquid is delivered through one or more nozzles or hollow needles with openings having diameters of the desired jet cross-section. The horizontal distance of the outlet openings from the outlet opening level of the ribbon nozzle and their elevation above the horizontal gelling stream are chosen as being so small that the junction of the two streams is bound to take place within the area where they are still laminar. After a certain distance, requisite for the solidification of the sol-gel globules, the gelling stream may become turbulent. The gelling stream carrying the fine globules is removed through a channel or duct. For continuous operation, the duct may be connected with a separating device, as in the system utilizing the aforedescribed dripping method, and the separated and regenerated gelling liquid may be returned to the ribbon nozzle.
For optimally producing fractions of sol-gel globules for sphere-pac nuclear fuels, therefore, both of the aforedescribed known processes are used. However, each of these methods requires rather high investments in equipment with a corresponding requirement of space. This entails a disadvantage, inasmuch as the sol-gel globules must be manufactured in glove-boxes which are kept as small as possible for reasons of economy, thereby resulting in a very cramped space. Thus, while these methods permit the satisfactory production of sol-gel globules for sphere-pac nuclear fuel of good quality, the production is very expensive because of the necessary equipment required.